Three-color sensor with a pin or nip series of layers

ABSTRACT

A pin or nip layer sequence, especially for use as a color sensor in electrooptical components. The bond gap of a first intrinsic (i) layer closer to the light input side is greater than the bond gap of a second i layer adjacent to the first and further removed from the light input side. The new μτ product for the i layer furthest distant from the layer is greater than the μτ product of an i layer closer is the n layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage of PCT/DE97/00713 filed Apr. 7,1997 and based, in turn, on German National Application 196 13 820.5 ofApr. 9, 1996.

FIELD OF THE INVENTION

The invention relates to a multi layer pin or nip structure with aplurality of i layers which are bounded on one side by a p layer and onthe other side by a n layer. The invention further relates to anelectrooptical component containing such a structure, especially amulticolor sensor, on the basis of a pin or nip structure according. Theinvention also relates to a process for producing such a structure.

BACKGROUND OF THE INVENTION

The components known in of the art based upon amorphous silicon or itsalloys are comprised of two antiserially-arranged pin or nip diodes forforming a pinip structure or a nipin structure. The diodes are herearranged substantially perpendicular to the incident light direction.

Such an nipin structure having a photosensitive electronic componentbased upon amorphous silicon is known for example from U.S. Pat. No.5,311,047. It is known to optimize such a nipin structure with referenceto the spectral sensitivity dependent on the voltage applied to thisstructure.

The incorporation of additional intrinsically conducting layers on bothsides of the p-doped layer with a band gap between 1.74 eV and 1.9 eV,can improve the blue/green sensitivity in of an nip structure in theincident light direction or the red/green separation in a following pinstructure in the incident light direction. (Q. Zhu, H. Stiebig, P.Rieve, J. Giehl, M. Sommer, M. Bohm, "NEW TYPE OF THIN FILM IMAGESENSOR" in Sensors and Control for Advanced Automation, edited by M.Becker, R. W. Daniel, O. Loffeld, Proc. SPEI 2247 (1994) 301).

It is known to absorb blue light preferentially in the firstintrinsically-conducting (i) layer, to absorb green light in the frontpart of the intrinsically-conducting layer and rear diode and to absorbred light by reduction of the μτ product in the rear part of the secondintrinsically-conducting layer. For that purpose a nipin structure isused in which one or more additional intrinsic layers are provided. (H.Stiebig, J. Giehl, D. Knipp, P. Rieve, M. Bohm, AMORPHOUS SILICON THREECOLOR DETECTOR in MRS Symp. Proc. 377 (1995) 813 or H. Stiebig, C.Ulrichs, T. Kulessa, J. Folsch, F. Finger, H. Wagner, TRANSIENTPHOTOCURRENT RESPONSE OF A-SI:H BASED THREE COLOR NIPIN DETECTORS, inICAS 16, Kobe, Japan, Sep. 4-8 1995). Here τ is the lifetime of thecharge carrier generated by the instant light and μ is its mobility.

A disadvantage of the known components described is that only up tothree linear independent structural sensitivity paths can be detectedwith this construction of the layer system. As a result a comparativelyexpensive superposition of these three independent signals is requiredfor color image processing.

OBJECTS OF THE INVENTION

It is the object of the invention, therefore, to provide a structurewith a plurality of i layers in which the independent signals are soobtained that less superposition is required to the point that it ispossible to avoid such superpositioning altogether.

It is also an object of the invention to provide an economicallyfabricated optoelectronic component, especially a multicolor sensor, thespectral sensitivity of which can be continuously shifted by variationof the applied voltage and thus a spectral sensitivity with desiredboundary conditions can be obtained for the detection of more than threelinearly independent curves or graphs of the spectral sensitivity in theincident light range from UV to near IR.

SUMMARY OF THE INVENTION

These objects are attained with a structure wherein a plurality of ilayers are bonded on one side with a p layer and on the other side withan n layer. These i (intrinsically conducting) layers are formed so thatthey constitute:

in the incident light direction at least in the boundary region betweentwo neighboring i layers (i^(I), i^(II)), a band gap Eg(I) of therespective first i layer (i^(I)) at the side neighboring the light inputside, which is greater than a band gap E_(g) (II) of the secondneighboring i layer (i^(I)) which is further away from the light inputside, and

in the direction of the p to n layer at least in the boundary region oftwo neighboring i layers (i^(I), i^(II)) a product μτ (I) which isgreater for the i layer (i^(I)) further away from the n layer than theproduct μτ (II) of the i layer (i^(II)) more closely neighboring the nlayer.

It has been found that, for broadening the functionality of a devicecontaining such a structure, from the incident light directionoutwardly, as a rule the respective band gap E_(g) of the individual ilayers is reduced by a suitable choice of the alloys, while thetransport characteristics (which can be characterized with the aid ofthe product μτ) of the photogenerated charge carriers for a nipinstructure is improved in both intrinsic layer systems of the top diodeand bottom diode in the direction of the p layer. For a pinip structure,as a rule, the band gap of the individual layers, can be reduced by asuitable choice of the alloy from the light impingement direction aswith the nipin structure while the μτ product rises from the central nlayer to both p conducting layers.

By stating, "as a rule" it is meant that it has been found that at leastin the boundary region between two neighboring i layers the formulationin the main claim for the band gap and the transport characteristics arefulfilled. Thus in an advantageous manner within the respective i layerthe value of the band gap and the transport characteristics are heldconstant. It is, however, conceivable to permit a limited variation ofthe respective values within the respective i layer.

Furthermore, the collection of the charge carriers by a change in theelectric field (for example by microdoping the i layer) can beinfluenced so as to permit the spatial charge carrier collection to bevaried in this manner.

It has been found, in this connection, that the structures are notlimited to structures of the nipin or pinip type. Rather, and alsoencompassed within the invention are components containing suchstructures of a simple pin or nip type. It appears to be sufficient withsuch a structure that a single diode polarity be provided by contrast tothe antiserially connected double diodes of the pinip or nipin structuretypes commonly thought to be necessary.

By variation of the voltage applied to pin, nip, pinip or nipinstructures according to the invention, the spectral sensitivity of thesestructures or components can be shifted in the range from the farultraviolet (UV) to the near infrared (IR) and the desired correspondingboundary conditions can be selected.

Advantageously, the structure according to the invention is so providedthat it allows continuous adjustment of the voltage applied to thestructure and thus detection of different wavelengths by such variation,thereby at least reducing the requirements for superposition which hashitherto been necessary.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features, and advantages will become morereadily apparent from the following description, reference being made tothe accompanying drawing in which:

FIG. 1 is a diagram showing the spatial distribution of the band gapE_(g) and the μτ product of a nipin structure according to the inventionfor example suitable as a color sensor;

FIG. 2 is a graph of the spectral sensitivity versus wavelengths of anipin structure according to FIG. 1;

FIG. 3 is a graph the defect density N_(D) as a function of the opticalband gap for a-Si_(1-x) Ge_(x) :H, whereby in the inset, the opticalband gap is shown as a function of the germanium content in thematerial;

FIG. 4 is a graph of the photoconductivity ((∘) at AM 1.5) and darkconductivity (•) σ as a function of the optical band gap E_(g) fora-Si_(1-x) Ge_(x) :H;

FIG. 5 is a graph of the ambipolar diffusion length L_(ambi) as afunction of the optical band gap E_(g) for a-Si_(1-x) Ge_(x) :H bycomparison to values known from the literature, namely, YOUNG et al,BAUER et al and TSAO et al;

FIG. 6 is a graph of the photoconductivity σ as a function of theoptical band gap E_(g) for a-S_(1-x) Ge_(x) :H made from asilane/methane mixture with hydrogen dilution (∘) and without hydrogendilution (•), squares according to MATSLIDA et al;

FIG. 7 is a diagram showing the effect of microdoping on the electricfield for a nip structure with three i-layer regions I, II and III whichhave different transport properties (μτ^(I), μτ^(II), μτ^(III)) for thecharge carriers;

FIG. 8 is a diagram in the form of a band gap profile and showingvariation of the transport characteristics of a color sensor based on apin structure;

FIG. 9 is a graph of the voltage dependent spectral sensitivity of aunipolar color sensor on the basis of a pin structure with three I layerregions; and

FIGS. 10A and 10B are diagrammatic sections through layer structuresaccording to the invention.

DESCRIPTION AND SPECIFIC EXAMPLES

In FIG. 1 the cross section of a nipin type structure is shown with atop diode and a bottom diode formed one after the other in the lightincidence direction (from left to right). The upper illustration showsthe curves of the band gap E_(g), the lower illustration the curves ofthe transport characteristic μτ as a function of the location coordinatex in the layer sequence.

The top diode contains a nip structure with two i layers i^(I) andi^(II). These are so formed that they have a band gap Eg^(I) =2.0 eV andEg^(II) =1.95 eV, respectively. These layers are fabricated with the aidof a silane-methane mixture in a ratio of 20 sccm/20 scem for i^(I) and4 sccm/(4 sccm with 180 sccm H2) for i^(II). The individual i layers ofboth the top and bottom diodes had constant values for E_(g) and μτ.

The rear bottom diode contains a pin structure with three i layersi^(III), i^(IV) and i^(V). These are so formed that they have a band gapof Eg^(III) =1.9 eV, Eg^(IV) =1.65 eV and Eg^(V) =1.50 eV. These ilayers i^(III), i^(IV) and i^(V) are prepared of amorphous Si(C),Si_(1-x) Ge_(x) (with x=0.5) or Si_(1-x) Ge_(x) (with x=0.20).

In FIG. 2 an example of a five-color sensor with a structure accordingto FIG. 1 is demonstrated which has a positive high voltage (the frontdiode is in the blocking direction) which sets the spectral sensitivityfor a wavelength of 470 nm (collection of all generated charge carriersin the front top diode) with a shift to 500 nm for lower positivevoltages.

With different μτ products in the intrinsic layers of the top diode,with smaller positive voltages, the photogenerated electron-hole-pairsare collected in the rear part of the intrinsic i layer (regionμτ^(II)), while the generated carriers recombine in the region of thereduced μτ product.

The different μτ products are obtained for example by the use ofmaterials which are fabricated with different hydrogen dilutions. Theelectric field rises only upon the application of higher positivevoltages above the top diode and the charge carrier pairs can then becollected in the front region of the intrinsic i layer of the top diode.

For negative voltages or for the rear bottom diode, three differentregions are realized with different μτ products. By applying negativevoltages, the bottom diodes can be blocked and the photogenerated chargecarriers can be collected in this diode. For small negative voltages,the charge carrier pairs are collected in the regions (μτ)^(III)(maximum of the spectral sensitivity at 550 nm), while they recombine inall other regions (μτ)^(IV) and (μτ)^(V) by the employment of materialwith reduced life time. If one increases the electric field above theintrinsic layer in the bottom diode, one can collect the charge carriersin the regions with (μτ)^(III) and (μτ)^(IV), while the photogeneratedcharge carriers are recombined further in the region with (μτ)^(V)(maximum of the spectral sensitivity at 595 nm).

With still higher applied negative voltages, all photogenerated chargecarriers collect in the bottom diode and a further maximum in thespectral sensitivity at 650 nm is observed. Thus, for example withvoltage values of +3V, 1V, -1V, -2V and -4V, respective linearlydependent curves of the spectral sensitivity can be detected.

With many other i layer regions with respective different transportcharacteristics (μτ products) a substantially continuous voltagedependent spectral sensitivity can be realized. In this case, thestructure with its very many i layers with respective different bandgaps and transport characteristics becomes equivalent to a structurewith one i layer and within which there are variable band gaps and/ortransport characteristics. This embodiment also falls within the subjectmatter of the appended claims and as such is claimed.

The transport characteristics can be adjusted or varied by one or moreof the following features given by way of example:

a) incorporation of germanium (Ge), carbon (C), nitrogen (N) or oxygen(O) in the amorphous silicon (a-Si) lattice, for example a-SiGe:H,a-SiC, a-SiH, a-Si_(1-x) N_(x) :H or a-Si_(1-x) O_(x) :H;

b) introduction of hydrogen or variation of the hydrogen concentrationin the process gases both for a-Si:H and for its alloys as mentionedunder point a);

c) variation of the process parameters as, for example, pressure,temperature or power.

The fabrication conditions can have the following effect on thetransport characteristics given in detail of an a-Si:H based alloy:

The optoelectronic characteristics of amorphous silicon (a-Si:H) can beinfluenced by variation in the fabrication conditions. For example, thematerial quality can be changed by variation of the deposition pressure,the temperature, the supplied electric power or by suitable addition offurther process gases (e.g. hydrogen, helium, argon, fluorine) in thelayer deposition. This means that the charge carrier transportcharacteristics (i.e. the product of charge carrier life and chargecarrier mobility as well as the ambipolar diffusion length) can beadjusted in a targeted manner.

For amorphous silicon alloys, as for example in silicon-germanium(a-Si_(1-x) Ge_(x) :H) and silicon-carbon alloys (a-Si_(1-x) C_(x) :H)the transport characteristics change in part very to strongly just as aresult of the composition of the respective alloy. This result has beenillustrated for example for the silicon germanium alloy in FIGS. 3-5.

With increasing germanium content, the optical band gap can becontinuously adjusted between E_(G) ≈1.8 eV (a-Si:H) and E_(G) ≈1.0 eV(a-Ge:H) as can be seen from the insert in FIG. 3. With increasing Gecontent the defect density increases as detected by the measuring set upknown as the "constant photo current method" by two orders of magnitudeor more (FIG. 3 large Figure). Simultaneously, the photoconductivity andthe dark conductivity change (see FIG. 4) as well as the ambipolardiffusion length (see FIG. 5) in dependence of the alloying composition.

The comparison in FIG. 5 is based upon the following references:

W. Luft, Y. S. Tsuo: Hydrogenated Amorphous Silicon Alloy DepositionProcesses, Marcel Dekker Inc., New York, Basel, Hong Kong, (1993).

G. H. Bauer, C. E. Nevel and H. -D. Morhing, Mat. Res. Soc. Symp. Proc.118, 679 (1988).

L. Yang, L. Chen, A. Catalano, Mat. Res. Soc. Symp. Proc. 219, 259(1991). The photoconductivity is then proportional to the product of thecharge carrier life τ and mobility μ and mirrors the transportcharacteristics of the majority charge carriers (here electrons). Theambipolar diffusion length identified material substantially thetransport characteristics of the minority charge carriers (here holes).Furthermore, (as in the case a-Si:H), at silicon alloys preparativeapproaches can be used (as have been described above), especially bysupply of additional process gases during the deposition to influencethe transport characteristics [Matsuda 1,2]. As an example for the alloysystem a-Si_(1-x) C_(x) :H, the influence of hydrogen addition("hydrogen dilution") to the process gases silane (SiH₄) and methane(CH₄) has been given in FIG. 6. (the comparison is based upon thefollowing reference:

J. Folsch, Dissertation, Universitat Stuttgart 1995, in: Berichte desForschungszentrums Julich, Jul-3147 (1995). That material made withgreater hydrogen dilution has a substantially greater photoconductivityand hence higher values for the μτ product than is the case where thematerial is deposited without H₂ supply. This effect sharply increaseswith increasing C (carbon) content in the material. The ratio of [H₂ ]to [SiH₄ ] + ([CH₄ ]) can assume in this case values for example of 10to 50.

From the literature and prior art relating to pin solar cells, it isknown that the collection of blue light in pin diodes can be reduced bya doping gas drag (as for example boron) in front regions and increased(for example) by a phosphorous doping in the i layer. By the combinationof microdoping and simultaneous variation of the band gap E_(g) and thetransport characteristics in the individual i layers, spectral colorseparation can be further improved. This will be described still furtherin connection with the top diode (nip structure) of a nipin structure(FIG. 7, FIG. 8).

FIG. 7 shows the curves [graph] of the band gap Eg and the transportcharacteristics μτ as a function of the coordinates of the nip structureaccording to the invention with three i layers.

By incorporation of phosphorous (FIG. 7, curve 1) a positive locallyfixed charge is incorporated in the absorber layers. Thus the electricfield in the region of p/i transition is raised while it is reduced inother parts of the absorber layer (in the region of the i/n transition).Thus the locally dependent collection is supported by the localdependency of the electric field distribution. The opposite fact isobserved for the incorporation of boron (FIG. 7, curve 2).

It will be self understood that corresponding optimization of the colorseparation, for example by the incorporation of phosphorous, can beobtained also in the bottom diode of a nipin structure or in theintrinsic i layers of a pinip structure.

Furthermore, with the aid of partial doping, individual i layers of themodified absorber layer (with example boron, phosphorous, arsenic orfluorene) the field and, therefore, the wavelength dependent collectionefficiency can also be altered or adjusted.

For multicolor sensors based on a nip structure or pin structure, as canbe deduced for example from FIG. 7 or FIG. 8, the following applies:

A nip or pin structure suitable as a photodiode according to theinvention, whose spectral sensitivity can be shifted by the variation ofthe applied voltage, enables the formation of a unipolar detector. Inthis case only a single polarity characterizes the voltage applied tothe structure for the detection of the three known colors (red, greenand blue) or other colors or, where appropriate, a plurality of thesecolors are to be detected.

Since with the structure of the invention in for example a bottom diode,three linearly independent equations apply to the detection of three ormore colors in the aforedescribed manner even one structure based on pinor nip layer sequence base with a respective plurality of i layers cansuffice. The pin or nip layer sequence with a respective plurality of ilayers according to the invention satisfies the objects set forth abovebetter than the characteristics of the known antiserial pinip or nipinlayers sequences of nor expensive construction. According to theinvention, it is possible to obtain also a unipolar three color sensoror multicolor sensor with a pin structure or nip structure basis,especially based on amorphous silicon or its alloys.

One such structure suitable for forming a sensor also has the advantagethat for the formation of a sensor array with the aid of such astructure there is a significantly reduced coupling of neighboringpixels since this structure in comparison to a nipin or pinip structurehas no conductive intermediate p layer or n layer neighbored on bothsides by i layers. In addition, for the applied voltage advantageouslyonly a single polarity is necessary.

In addition, the structure according to the invention has transientproperties which are improved with respect to a detector based on twoantiserial diodes because recharging processes of the stored charge inthe top diode or bottom diode do not arise as have been observed bypinip or nipin structures.

Advantageously and also desirable is the use of three or more i layerswith each having a respective but different μ96 product. To optimize thesystem, a microdoping can be carried out analogous to that with nipinstructures. For such doping, phosphorous, boron or arsenic can be used.

For a pin structure, the band gap E_(g) in the light incident directionis reduced at the boundary regions between neighboring I layers. Theabsorber layers can be subdivided into three regions with different μτproducts, whereby the transport characteristics of the material used ispoorer from the p conductive layer to the n conductive layer as has beenillustrated in FIG. 8. In FIG. 9 the measurement results for a structureaccording to FIG. 8 have been shown to include for a rising negativeapplied voltage, the shift in the spectral sensitivity and the targetedadjustability as a function of the wavelength of the light.

In a nip structure the band gap is also reduced from the lightimpingement direction (n layer) while the transport characteristics inthe three different regions improve from the n layer to the p layer.

To FIGS. 10A and 10B in which a linear sensor array or two dimensionalsensor array of the nipin or pin color sensor type schematically shown,the following applies. The high conductivity of doped n and p layersgives rise to coupling currents perpendicular to the light incidentdirection in unstructured arrays. The size of an active pixel in anunstructured sensor array is defined by its back contact (structuredmetallized plane). A coupling current (I_(q)) which thus arises issuperimposed on the photocurrent and hinders or obstructs the evaluationof the signal. Coupling processes via the n layer (last semiconductorlayer before the back contact), in the case of a nipin diode or pindiode (in a pinip detector, corresponding to the p layer) can be limitedby technological steps (for example inverted deposition on a structuredsubstrate in which the n layer breaks away from previously defined pixeledges) or by a special design of the read out electronics (U₁ =U₂ =U₃. .. ). The coupling via the p layer in a nipin detector (corresponding tocoupling via the n layer in a pinip diode) permits such a reduction onlyin the case of complete patterning (removal of the thin layer systembetween individual pixels) of the array or by the transition to aunipolar detector (pin or nip diode) where the complex i layer system isnot interrupted by a layer with high conductivity. Since patterning ofthe complete array is technologically expensive and cost intensive, theformation of an unstructured sensor array of pin or nip diodes whichallow separation of three or more spectral components of the light,represents an advance from the point of view of reduction in thecoupling current.

LITERATURE LIST

[1] A. Matsuda, T. Yamaoka, S. Wolff, M. Koyama, Y. Imanishi, H.Kataoka, H. Matsuura, K. Tanaka, J. Appl. Phys. 60, 4025 (1986).

[2] A. Matsuda, K. Tanaka, J. Apl. Phys. 67, 7065 (1990).

[3] W. Luft, Y. S. Tsuo in: Hydrogenated Amorphous Silicon AlloyDeposition Processes, Marcel Dekker Inc., New York, Basel, Hong Kong,(1993).

[4] G. H. Bauer, C. E. Nevel and H.-D. Morhing, Mat. Res. Soc. Symp.Proc. 118, 679 (1988).

[5] L. Yang, L. Chen, A. Catalano, Mat. Res. Soc. Symp. Proc. 219, 259(1991).

[6] J. Folsch, Dissertation, Universitat Stuttgart 1995, in: Berichtedes Forschungszentrums Julich, Jul-3147 (1995).

We claim:
 1. A structure with a plurality of i layers which are boundedon one side with a p layer and on the other side with an n layer, said ilayers constituting:in an incident light direction at least in aboundary region between two neighboring i layers (i^(I), i^(II)), theband gap Eg(I) of the respective first i layer (i^(I)) at respectivelythe side neighboring the light input side, which is greater than theband gap E_(g) (II) of the second neighboring i layer (i^(I)) which isfurther away from the light input side, and in a direction from the p tothe n layer at least in the boundary region of two neighboring i layers(i^(I), i^(II)) the product μτ(I) which is greater for the i layer(i^(I)) further away from the n layer than the product μτ(II) of the ilayer (i^(II)) more closely neighboring the n layer.
 2. A structureaccording to claim 1 having a pin or an nip structure.
 3. A structureaccording to claim 1 with a multiplicity of i layers (I, II, . . . ). 4.A structure according to claim 1 wherein the respective band gap E_(g)(I, II, . . . ) within each of the i layers has a constant value.
 5. Astructure according to claim 1 wherein the respective product μτ(I, II,. . . ) within each of the i layers has a constant value.
 6. A structureaccording to claim 1 wherein for forming a pinip structure, the n layeron a side neighboring the i layer has a further layer sequence with atleast one further i layer, each further layer sequence being bounded bya further p layer.
 7. A structure according to claim 1 wherein to form anipin structure the p layer on its side neighbored by the i layer has afurther layer sequence at least one further i layer, each further layersequences being bounded by a further n layer.
 8. An array with amultiplicity of structures according to claim 1 each with a respectivepixel forming structure.
 9. A structure according to claim 1 in the formof a multicolor sensor.
 10. A process for producing a structureaccording to claim 1 whereina structure with a plurality of i layers isformed which on one side is bounded by a p layer and on the other sideis bounded by an n layer, the i layers are selected so that in a lightincident direction at least in the boundary region of two neighboring ilayers a band gap E_(g) (I) of the first i layer which respectively moreclosely neighbors the light incident side is greater than the band gapE_(g) (II) of the second i layer which is further away from the layerneighboring the light intake side, and in a direction from the p layerto the n layer at least in the boundary region between two neighboring ilayers (i^(I), i^(II)) the product μτ(I) of the i layer further awayfrom the n layer is greater than the product μτ(II) of the i layer(I^(II)) more closely neighboring the n layer.